The HTML5 standard won't be finished until 2014, so the posturing at this point is mostly for public relations. The World Wide Web Consortium has said it's too early to draw broad conclusions from HTML5 tests, but that's exactly what browser makers are doing.

Microsoft said it has submitted nearly 4,000 HTML5 tests to standards bodies, and that users can try the tests themselves, but only on an official Microsoft website. Microsoft, which is also fighting a standards battle with Google over the codecs to be used in HTML5 videos, said this month that it has "the world's fastest browser." But Rouget begs to differ, saying it's no wonder Microsoft's own tests show IE9 knocking HTML5 out of the park.

"Does IE9 support 99% of the HTML5 specification as insinuated by Microsoft? No, they're actually pretty far from it," Rouget writes. "The tests Microsoft are referring to are the ones they created during the development of IE9. It's not that surprising that they pass the very tests they used to design and develop the browser, we score pretty well against our own unit-tests as well."

"The primary use case for these tests, however, is to spot regressions and validate code changes. In other words: the tests ensure that future changes don't break the things you just built. They don't actually test all elements of a specific standard."

Rouget pointed to a "real world test" on the site Caniuse.com, which he said shows Firefox 4 achieving 84% compatibility with all web standards, compared to 61% for IE9. He also pointed to HTML5test.com, which has Firefox 4 achieving a score of 255 to IE9's 130.

Those scores are based on development and beta versions. For stable browser versions (IE8 and Firefox 3.6.8), Microsoft scores just 32 and Firefox just 156. Google's Chrome, meanwhile, receives an HTML5 score of 283 for its stable version and 293 for the beta version, besting all desktop and mobile browsers.

But still another test puts IE9 in the lead. Former Linden Lab CTO Cory Ondrejka, now at Facebook, tested how HTML5 could apply to web-based games, and posted results showing IE9 outstripping Chrome, with Firefox, Opera and Apple's Safari well behind.

Mozilla's Rouget doesn't mention Chrome's HTML5 results, but does provide a list of 17 features he says aren't supported by IE9, including HTML5 forms, JavaScript Strict Mode, application cache, and drag and drop from the desktop.

"The reality is that IE9 is two years late," Rouget writes. "Microsoft is glad to come out with the (VIDEO) tag, the (CANVAS) tag, SVG, and some CSS3. Like other vendors did years ago. Firefox 3.5 had the (VIDEO) tag, the (CANVAS) tag, Geolocation, SVG in 2009. Canvas and SVG existed five years ago."

Rouget identifies only two IE9 features not supported in rival browsers, text-overflow and Calc. "That's why I don't consider IE9 a modern browser," he concludes.

In response to Rouget's blog, Microsoft released a statement saying:

"We spent much time researching and looking at what developers are building today and what they want to build tomorrow to define what we build in Internet Explorer 9 today. We used that research to inform our view of HTML5 and the pieces that matter most to designers and developers in building these next set of experiences on the web."

"In terms of HTML5 defined, it remains a combination of what developers will want to do and the definitions set forth by the W3C as the ultimate authority in defining standards for the web."